UV Imaging Method Peers into Ultrafast Semiconductor Structures

Researchers from the University of Jena have developed a method called coherence tomography with extreme ultraviolet light (EUV). The technique, which has applications in materials research and data processing, enables the study of the interior structures of semiconductor materials in a nondestructive way.

The imaging method is based on the operational principles of optical coherence tomography (OCT), an established imaging method in areas such as ophthalmology, doctoral candidate Felix Wiesner said. “These devices have been developed to examine the retina of the eye noninvasively, layer by layer, to create three-dimensional images.”

Professor Gerhard Paulus, Ph.D. student Felix Wiesner, and Dr. Silvio Fuchs (from left) in a laser lab of the Institute of Optics and Quantum Electronics at the University of Jena. Courtesy of Jens Meyer, University of Jena.

In ophthalmology applications, OCT uses infrared light to illuminate the retina. Light is selected in such a way as to prevent the tissue that is being examined from absorbing too much of it, and so that light may be reflected by the inner structures.

For their purposes, due to the size and structures the team hoped to image, the researchers chose extremely shortwave UV light instead of the longwave infrared light used for OCT. To examine semiconductor materials, the structures of which are on the nanometer scale, light with a wavelength of only a few nanometers is needed.

Previously, generating light with that short of a wavelength was possible only in large-scale research facilities, but with the advent of high harmonics, the researchers were able to generate light in an ordinary laboratory. The interaction of laser light with a medium produced radiation, creating light with a frequency many times greater than ordinary light. The higher the harmonic order, the shorter the resulting wavelength.

“In this way, we generate light with a wavelength of between 10 and 80 nm using infrared lasers,” said Gerhard Paulus, professor of nonlinear optics at the University of Jena. “Like the irradiated laser light, the resulting broadband extreme UV light is also coherent, which means that it has laser-like properties.”

To test the method, the researchers exposed nanoscopic layer structures in silicon to the coherent extreme UV radiation and analyzed the reflected light. The silicon samples contained thin layers of metals such as titanium or silver, at different depths. Because the reflective properties of these materials are different from those of the silicon, the team was able to detect them in the reflected radiation.

The method is precise enough to display the deep structures of the samples. The chemical composition of the samples can also be determined due to the variations in reflective behavior.

“This makes coherence tomography an interesting application for inspecting semiconductors, solar cells, or multilayer optical components,” Paulus said.

The method has further potential applications in quality control in the manufacturing of nanomaterials, where it could be applied to detect internal defects or chemical impurities.

The research was published in Optica (www.doi.org/10.1364/optica.412036).